Spinal surgeons often treat spinal disorders with spinal fusion augmented with elongated spinal rods connected to the spine with pedicle screws. Such “rod assemblies” generally comprise one or two spinal rods and a plurality of screws inserted through the pedicles and into their respective vertebral bodies. The screws are provided with connectors, for coupling the spinal rods to the screws. The spinal rods extend along the longitudinal axis of the spine, coupling to the plurality of screws via their connectors. The aligning influence of the rods forces the patient's spine to conform to a more appropriate shape.
Those bone screw connectors often employ tabs or towers to facilitate the assembly. Once the screws and rods are fixed in place, these tabs or towers are removed from the connector. Various means to remove the tabs have been used including releasable pins and frangible tear or fracture seams. The tabs or towers often called extension legs vary in length from short, an inch or less, to very long, four or more inches. The removal of these extension legs needs to be simple and reliable. Most preferably, the removal needs to be consistent. The attachment must be secure so as not to prematurely fail, but not so strong as to require large forces or multiple flexing to release the legs. Some prior art bone screws have resorted to separate tools to initiate a breakage of a connection due in part to the excessive forces required, as taught in U.S. Pat. No. 7,927,360.
In U.S. Pat. No. 8,858,605, a prior art bone screw system is disclosed that has a fixation element, a receiving element, coupling element, and a compression element. The fixation element was a screw. The receiving element defined an internal bore sized to receive the shank portion of the fixation element and had a seat adapted to support the head portion of the fixation element. The seat of the receiving element was shaped to substantially conform to an exterior portion of the head portion of the fixation element. In this prior art patent, the systems functional features are described as follows.
The receiving element was further adapted to receive a stabilizer rod. As such, in one aspect, the receiving element comprised a pair of opposed legs separated by a rod-receiving channel. This created a defined shape often referred to as a tulip in the orthopedic fastener art.
In another aspect, the bone screw system also comprised a pair of leg extensions. Each leg extension had a first end and a second end, where the second end was coupled to a respective opposed leg of the receiving element.
The compression element was engageable with the receiving element. In one aspect, the compression element was adapted to move downward into the compression element receiving chamber to translate a force to the stabilizer rod and translate a force onto the head portion of the fixation element and substantially fix the position of the fixation element with respect to the receiving element.
In one embodiment, a breakaway groove was positioned externally on each leg or leg extension. The breakaway groove extended inwardly at least partially through the wall of each leg or leg extension. This created an aperture or slit in the wall leaving the wall with at least one, preferably a pair of leg attachments connecting each leg to each respective leg extension adjacent the aperture.
In a preferred embodiment, the leg extensions also had threads continuing from the legs along its length at distances equal or less than the length of the leg extension. The threads accepted the external threads of the compression element. The wall thickness of the legs or leg extension was a minimum (t) at the thread groove and the breakaway groove extended to at least a portion of a thread groove at the distance of (t) or greater. The internal threads of the legs and leg extensions had a pitch so the threads formed a helix angle and the breakaway groove intersected the pitch along at least one thread or thread groove.
The breakaway groove was oriented perpendicular to an axis of the receiving channel.
All of these early systems had an undesirable feature that although allowed the bone screw to be polyaxial relative to the connector, it freely moved or flopped about on insertion making it hard to control. The present invention solves this issue and affords the surgeon a much easier to use bone screw system assembly.
These and other objectives are achieved by the invention as described hereinafter.